01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, mix ground turkey, breadcrumbs, Parmesan, egg, garlic, parsley, oregano, salt, and pepper until just combined.
03 - Shape mixture into sixteen 1 1/2-inch meatballs and arrange on the prepared baking sheet.
04 - Bake for 15 to 18 minutes until cooked through and lightly browned.
05 - Heat olive oil in a saucepan over medium heat. Sauté onion until softened, about 3 to 4 minutes; add garlic and cook one more minute.
06 - Stir in marinara sauce, basil, and red pepper flakes. Simmer on low heat for 10 minutes, stirring occasionally.
07 - Add baked meatballs to the sauce and cook together for another 5 minutes.
08 - Split sub rolls and spread with softened butter. Toast under broiler or in a skillet until golden.
09 - Place four meatballs with sauce into each roll and top evenly with mozzarella cheese.
10 - Place assembled subs under the broiler for 2 to 3 minutes until cheese is bubbly and melted.
11 - Garnish with chopped fresh basil or parsley if desired and serve hot.